Resonant cavity structures



Nov. 11, 1958 I A. J. YOUNG ETAL 2,860,281

RESONANT CAVITY STRUCTURES Filed May 11. 195.4

zi w w p xmmav [0457 48- United States Patent RESONANT CAVITY STRUCTURESJohn Ralph Bagnall, to English Electric a British Arthur James Young,Danbury, and

Chelmsford, England, assignors Valve Company Limited, London, England,company This invention relates to resonant cavities and morespecifically to resonant cavities of the kind adapted to co-operate astuned circuits with associated electron discharge tubes for ultra highfrequency working.

For the sake of brevity andclarity of description the invention will beparticularly described herein as applied to resonant cavities adapted toco-operate with associated so-called klystron tubes but, as will beobvious later, the invention is not limited to this particularapplication but may be used to advantage in all cases in which aresonant cavity is required to be associated with an electron dischargetube.

One important use of the invention-not, by any means, the only -one-isin radar apparatus. In many marine radar installations, and in othermicro-wave equipment, there is a fixed frequency magnetron oscillator incooperation with a klystron local oscillator, which'is employed toheterodyne the oscillations from the magnetron to a predeterminedintermediate or beat frequency for superheterodyne reception. So far asthe user of such equipment is concerned, his requirement as respects thetuning of the klystron local oscillator is merely that he must be ableto tune it over the comparatively narrow band of frequencies over whichthe oscillations from a given magnetron, under given operatingconditions, are in practice scattered. Unfortunately, however, due toinherent manufacturing difficulties, the manufacturer of a klystron,especially of a klystron for use on the shorter wave lengths, mustinvariably make his tube to accommodate a much wider spread offrequencies; in fact the maximum spread of frequencies from magnetron tomagnetron in any specific equipment is, in practice, only a smallfraction of that which the manufacturer of a klystron must accommodate.

For this reason it is common practice to provide two tuning adjustmentcontrols for a klystron oscillator or other device using an evacuatedelectron discharge tube in conjunction with a tuning resonant cavity.The first is what may be termed a manufacturers control-i. e. anadjustment which is intended to be made by the manufacturer or by amaintenance engineer--and the second is what may be termed a userscontroli. e. a control actuated by a knob or handle which isconvenientto the user. The manufacturer uses the first control to adjustthe center frequency to a value at which the second control will tuneover the range required when used in the equipment in question.

In the case of a klystron oscillator the manufacturers control isexercised upon the grid spacing, variation of which, of course, producesvariation of the effective capacitance of the resonant cavity associatedwith the tube. The grid spacing is exceedingly small and a very smallchange in the gap produces a large change in capacitance. To quotepractical figures, in the case of a klystron operating in the X-band achange of one thousandth part of an inch in the gap produces a frequencychange of the order of 100 mc./s. This form of control involves that thewhole resonant system must be ice enclosed within the evacuated envelopeof the tube with the result that changes in external atmosphericpressure cause consequent minute, but none the less serious, changes inthe relative mechanical disposition of parts within the envelope. Insome cases the manufacturers control is in the form of a capacityelectrode which can be adjusted across the cavity resonator and is setto provide the desired initial frequency. This control may be regardedas a trimmer capacity control auxiliary to the main capacity which isthe grid gap. In practice, with an X-band klystron, this control willvary frequency by about 30 mc./s. per thousandth part of an inch. Theusers control is generally a variable inductance tuner.

The present invention seeks to provide a capacity tuning adjustment fora resonant cavity which will give a relatively wide range of control,does not involve locating the cavity or the adjustment member thereofwithin the evacuated envelope, which can be readily adjusted by themanufacturer or by a maintenance engineer and left ad justed for a longperiod-normally for the life of the associated tuba-and which is simple,durable and cheap to manufacture.

According to this invention a resonant cavity structure adapted toco-operate as a tuned circuit with an electron discharge tube for ultrahigh frequency working comprises a first electrode forming part of saidstructure and positioned to be in capacity coupling relationship with anelectrode of a tube inserted in its normal position in said structure, asecond electrode behind said first electrode (i. e. behind when viewedfrom said tube electrode) and means for adjusting the relative positionof said first and second electrodes to cause the second one to projectto an adjustable extent from behind the first one.

Preferably the first electrode is fixed and the second electrode isslidable over the back thereof.

Preferably also the second electrode is carried by a deformable memberanchored to the cavity structure, means being provided to deform saidmember to adjust the position of the second electrode.

The invention is illustrated in the single figure of the drawing whichis a part sectioned elevation of one embodiment of the invention asapplied to a klystron tube and associated cavity resonator.

Referring to the drawing a klystron tube lt of the well known disc sealtype is soldered into position in a cavity resonator structure 2. Theklystron tube has the usual bottom and middle connector discs 3 and 4respectively which are soldered in position in correspondingly shaped.apertures in the walls of the cavity structure. The upper fiat wall 5 ofthe cavity structure is provided. with a ringlike electrode 6constituting a first capacity electrode extending partly across thecavity and in capacity coupling relationship with the inserted tube 1 toprovide a fixed capacitance. This electrode 6 is, as will be seen, partof the structure of the top of the resonator structure 2. Behind theelectrode 6 is a second capacity electrode or an adjustment 7 which iscontrollably slidable up and down (in the figure) over the back of thefirst capacity electrode 6 and in direct contact therewith. As it slidesdown its extent of projection beyond the lower edge of electrode 6 isvaried. It is carried by a deformable member constituted by a ringdiaphragm 8 whose outer edge is soldered to the top of the resonator or,as shown, clamped between the said top and the rest of the resonator.Means such as screws or bolts 9 with lock nuts 10 are provided to deformthe diaphragm 8 and thus adjust the electrode- 7 over the back ofelectrode 6.

The arrangement illustrated gives a very fine adjustment and in the caseof an X-band klystron a tuning rate of about 30 mc./s. per thousandthpart of an inch movement of electrode 7 while a wide range of coverageis provided. In practice, adjustment is effected until the nor- U mallyprovided inductive tuner (not shown) covers the required band.

The invention has the advantage that it does not require the provisionof any micro-wave chokes.

In manufacture the position of the electrode 6 is chosen to give aninitial frequency near but a little above the top,

frequency of the intended band of use. If this is done the device willstill operate satisfactorily if a given klystron tube is replaced (atthe end of its life) by another of the same design and type: in otherwords, normal manufacturersvariations between klystron tubes of the samedesign are accommodated.

While we have described our invention in one of its preferredembodiments, we realize that modifications may be made, and We desirethat it be understood that no limitations upon our invention areintended other than may be imposed by the scope of the appended claims.

We claim:

1. A resonant cavity structure adapted to co-operate as a tuned circuitwith an ultra high frequency electron discharge tube comprising incombination with a resonant cavity structure a first capacity electrodein the form of an annulus which extends partly across the cavity, asecond annular shaped capacity electrode radially behind the firstcapacity electrode and fitting close to the latter so that the innersurfaces of both electrodes are in sliding and good electrical contacttherewith, said first and second capacity electrodes being adapted toform effectively a single capacitive electrode capacitively coupled tosaid discharge tube which constitutes the second capacitive electrode,and controllable means operable from outside said structure foradjusting the relative position of the first and second capacityelectrodes whereby the second electrode can project to an adjustableextent from radially behind said first capacity electrode with respectto said discharge tube so as to vary the total effective area of saidsingle capacitive electrode relative to said discharge tube.

2 A resonant cavity structure as set forth in claim 1, wherein the firstcapacity electrode is fixed and wherein the second capacity electrode isslidable over the back thereof relative to the discharge tube.

3. A resonant cavity structure as set forth in claim 1, which includes adeformable member carrying said second capacity electrode and anchoredto the cavity structure, said controllable means including means fordeforming said member to adjust the position of the second capacityelectrode.

4. A resonant cavity structure for co-operating as a tuned circuit withan ultra high frequency electron discharge tube, the structurecomprising a first capacity electrode of cylindrical form and encirclingthe tube, the first capacity electrode forming part of said structure, asecond capacity electrode of cylindrical form and encircling said firstcapacity electrode and in good electrical contact therewith, said firstand second capacity electrodes being adapted to form effectively. asingle capacitive electrode coupled to said discharge tube, a,deformable ring diaphragm, the outer periphery of which is fiXed to saidstructure and the second capacity electrode being carried by thediaphragm at the inner periphery thereof, and means for deforming saidmember to adjust the extent to which said second capacity electrodeprojects from behind said first capacity electrode.

References Cited in the file of this patent UNITED STATES PATENTS

